Image pickup apparatus with light emission control adapted for light-receiving characteristics

ABSTRACT

An electronic flash light is emitted from a camera by causing light-emitting diodes corresponding to red, green and blue simultaneously. Light emission of the light-emitting diodes corresponding to respective colors is controlled in accordance with the light-receiving characteristics of the light-receiving elements. For example, by adjusting the on and off duration ratio of switches in accordance with the light-receiving characteristics, the quantity of light emitted by the light-emitting diodes is adjusted.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image pickup technology in whichlight-emitting elements produce an electronic flash light when pickingup an image.

2. Description of the Related Art

A digital camera picks up an image of a subject by a photo detector suchas a charge-coupled device (CCD). A CCD is provided with a photo diodethat converts outdoor daylight into electric charges by thephotoelectric effect. Charges generated according to the intensity oflight record image information, i.e. performs imaging. A CCD itself isan element sensing the intensity of light and cannot capture colorinformation of a subject image.

For this reason, a colored filter is usually provided for one CCD. Forexample, a color filter corresponds to, for example, a primary colorsuch as red, green and blue (RGB). The color filter has the property oftransmitting only the color it corresponds to. By combining CCDs andcolor filters, a digital camera is capable of capturing colorinformation of a subject as well as the intensity of light.

In recent years, as the pixel resolution of a CCD is improved, many ofbattery-driven portable appliances such as portable telephones andpersonal data assistants (PDA) now have a built-in digital camerafunction. Many of these appliances use light-emitting diodes (LED) toprovide backlight of a light-emitting device such as a liquid crystaldisplay (LCD) and an electronic flash.

[Patent Document No. 1]

Japanese Laid-Open Patent Application No. 2002-116481

SUMMARY OF THE INVENTION

In capturing color information of a subject image, it is to be notedthat the photo sensitivity of a CCD differs from color to color. Forexample, a CCD corresponding to blue and a CCD corresponding to yellowreceiving light of the same quantity do not necessarily generate thesame amount of charges. This is because a color filter does not transmitthe entirety of light and the transmittance thereof is different fromcolor to color.

The present invention is done in consideration of the problem discussedabove and its object is to-provide an image pickup technique in which animage true to a subject's original color is taken using an electronicflash by driving light-emitting elements in consideration of thecharacteristics of light-receiving elements.

The image pickup apparatus according to the present invention isprovided with a plurality of light-emitting elements corresponding torespective colors, an image pickup unit and a light-emitting circuit fordriving the plurality of light-emitting elements so as to provide anelectronic flash light. The light-emitting circuit generates anelectronic flash light by adjusting the quantity of light emitted by thelight-emitting elements in accordance with the light-receivingcharacteristics of the image pickup unit.

In accordance with the present invention, it is possible to pick up animage true to a subject's original color by emitting an electronic flashlight considering the characteristics of the light-receiving elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of a digital camera according to anembodiment of the present invention.

FIG. 2 shows how emission of light is controlled.

FIG. 3 shows a construction of a light emission control circuit.

FIG. 4 shows a light emission control chip in which a circuit fordriving the light-emitting elements is built into one chip.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a functional block diagram of a digital camera according to anembodiment of the present invention. The digital camera 100 includes aflash processing unit 110, an image pickup and processing unit 120, anLCD monitor 140, an operation unit 150, an optical sensor unit 160 and aprocessing block 200.

The image pickup and processing unit 120 picks up and processes imagesin response to an instruction from the processing block 200. The imageprocessing unit 120 includes a received light processing unit 122, anA/D converter 124 and a compression processing unit 126. The receivedlight processing unit 122 forms an image of a subject by receiving lightfrom the subject, and converts the formed image into an electricalsignal. The received light processing unit 122 includes a lens, a CCDand a color filter (not shown). The A/D converting unit 124 converts theelectrical signal into a digital signal. The compression processing unit126 compresses the digital image data of the subject.

The flash processing unit 110 controls emission of a flash light inresponse to an instruction from the processing block 200. The operationunit 12 includes a power switch, a release switch and the like thatallow the user to pick up images and set various operation modes. TheLCD monitor 140 displays image pickup/play modes, a zoom scale, date andtime and the like in addition to displaying the image of the subject. Anoptical sensor unit 160 senses ambient light. The optical sensor unit160 senses light of a predetermined quantity from a predeterminedorientation, and notifies the processing block 200 accordingly. Theprocessing block 200 makes a determination as to whether emission of aflash light is necessary by referring to the light detected by theoptical sensor unit 160.

The processing block 200 includes an imaging mode setting unit 220 and acommunications unit 222 in addition to the process control unit 210. Theprocess control unit 210 controls the whole process of the digitalcamera 100. The process control unit 210 includes a CPU (CentralProcessing Unit) 212 and a memory 214. The imaging mode setting unit 220sets various imaging modes. The term “imaging mode” refers to a set ofsettings related to image pickup. The communications unit 222 controlscommunication with external devices. The digital camera 100 need not bean appliance designed only as a digital camera but may be digital cameracapabilities provided in a portable telephone, a personal digitalassistant (PDA) and the like.

FIG. 2 shows how emission of light is controlled. FIG. 2 generallycorresponds to blocks such as the flash processing unit 110 and the LCDmonitor 140. A shared boost converter 300 boosts an input voltageaccording to a switching strategy, the input voltage being a batteryvoltage Vbat of a lithium ion battery 230, and outputs a preparatoryboosted voltage Vod. The shared boost converter 300 is connected to ablue light-emitting diode 400 a and a green light-emitting diode 400 b.The preparatory boosted voltage Vod output from the shared boostconverter 300 is fed to these LEDs. Each of these LEDs uses thepreparatory boosted voltage Vod as a driving voltage. Alternatively,each of these LED boosts or lowers the preparatory boosted voltage Vodto turn it into a driving voltage.

The red light-emitting diode 400 c is directly connected to the lithiumion battery 230. The red light-emitting diode 400 c is driven by abattery voltage Vbat lower than the preparatory boosted voltage Vod.This is because the forward voltage of the red light-emitting diode 400c is lower than that of the blue light-emitting diode 400 a and thegreen light-emitting diode 400 b. The blue light-emitting diode 400 a,the green light-emitting diode 400 b and the red light-emitting diode400 c are generically referred to as light-emitting diodes 400. Theboost chopper circuit 150 charges energy in and discharges energy from acoil L by an on and off operation of the transistor Tr1, thus boostingthe battery voltage Vbat to a preparatory boosted voltage Vod. A draincurrent flows through a resistor R3 via the coil L while the transistorTr1 is turned on in the boost chopper circuit 350, allowing the batteryvoltage Vbat to store magnetic energy in the inductance 114. When thetransistor Tr1 is turned off subsequently, the magnetic energy stored inthe coil L while the transistor Tr1 is turned on is discharged aselectric energy and turns into a current that flows through a Schottkybarrier diode SBD. The voltage generated in the coil L is positivelysuperimposed on the battery voltage Vbat and stabilized by a smoothingcapacitor Cl before being output as the preparatory boosted voltage Vod.

The boost ratio of the preparatory boosted voltage Vod output by theboost chopper circuit 350 is determined by a duration ratio between anon period and an off period of the transistor Tr1 operated as a switch.A pulse width modulation (PWM) circuit 330 produces the on and offduration ratio of the switch. Assuming that the on and off switchingperiod of the switch is T and the on duration of the switch is Ton, thePWM circuit 330 generates a pulse signal with a duty ratio of Ton/T. Adriver 340 turns the transistor Tr1 on and off in response to the pulsesignal generated by the PWM circuit 330. When the pulse signal is at ahigh (H) level, the transistor Tr1 is turned on. When the pulse signalis at a low (L) level, the transistor Tr1 is turned off.

The pulse width of the pulse signal generated by the PWM circuit 330varies in accordance with an output from the error amplifier 302. Theerror amplifier 302 compares a detected voltage Vs derived from dividingthe preparatory boosted voltage Vod by two voltage dividing resistors R1and R2 with a reference voltage Vref from a reference voltage source,and amplifies an error between the reference voltage Vref and thedetected voltage Vs so as to feed a resultant voltage back to the PWMcircuit 300. The PWM circuit 330 modulates the pulse width of the pulsesignal according to the output of the error amplifier 302 by controllingan on duration Ton of the switch, and matches the detected voltage Vswith the reference voltage Vref by feedback control.

The light-emitting diodes 400 provide backlight for the LCD monitor 140.The light-emitting diodes 400 also provide an electronic flash light fortaking photographs. Switches 414 are turned on and off in response to asignal from an external source (not shown). The switches 414 are turnedon so that the light-emitting diodes 400 are controlled by the PWMcircuit 410 for control of backlight emission. By controlling the bluelight-emitting diode 400 a, the green light-emitting diode 400 b and thered light-emitting diode 400 c to emit the same quantity of light, awhite light is produced. The PWM circuit 410 controls the quantity oflight produced by the individual LEDs by controlling a current fed tothe blue light-emitting diode 400 a, the green light-emitting diode 400b and the red light-emitting diode 400 c. By turning the switch 414 onand off in accordance with a signal from an external source (not shown),it is possible to control the quantity of light emitted by theindividual LEDs of the light-emitting diodes 400.

A light emission control circuit 500 is provided to cause the bluelight-emitting diode 400 a, the green light-emitting diode 400 b and thered light-emitting diode 400 c to emit an electronic flash light fortaking photographs. The present invention relates to a method ofcontrolling the light-emitting diodes 400 simultaneously to emit anelectronic flash light. The method will be discussed in detail byreferring to FIGS. 3 and 4.

The image pickup apparatus according to the embodiment of the presentinvention comprises a plurality of light-emitting elements correspondingto respective colors, an image pickup unit, a memory for storingparameters for driving the plurality of light-emitting elements to emita high-luminance electronic flash light for taking photographs and alight-emitting circuit for reading the parameters and driving theplurality of light-emitting elements in accordance with the parameters.The parameters are set so as to correct the light-receivingcharacteristics of the image pickup unit.

It is assumed that the blue light-emitting diode 400 a, the greenlight-emitting diode 400 b and the red light-emitting diode 400 c arecontrolled simultaneously to emit light and a white electronic flashlight is produced by combining the light of the respective colors. Evenif the CCD elements corresponding to red, green and blue receive thesame quantity of light, it is not ensured that the photodiodes of theCCD elements generate the same amount of electric charges. For example,when the photosensitivity of a CCD element corresponding to blue isgreater than the photosensitivity of a CCD element corresponding to theother colors, a bluish photographic image is taken even if a white flashlight is used to take a photograph.

It occurred to the inventor of the present invention that an image trueto a subject's original color is obtained by controlling the quantity oflight emitted by the light-emitting diodes 400 such that thelight-receiving characteristics of the CCD are corrected, instead of byproducing a pure white flash light. In relation to the example givenabove, it should be ensured that the quantity of light emitted by thegreen light-emitting diode 400 b and the red light-emitting diode 400 cis larger than that of the blue light-emitting diode 400 a. Given thatthe characteristics of the light-receiving elements are such that thephotosensitivity of the CCD element to a blue light is high, thecharacteristics of the light-receiving elements are corrected byadjusting the quantity of light emitted by the light-emitting elementsin such a way as to reduce the quantity of light emitted by the bluelight-emitting diode 400 a. As a result of this, a photographic imagetrue to a subject's original color is obtained.

Conventionally, a main target sought to be achieved in developing alight-emitting element is improvement in light-emitting performance.Anew viewpoint introduced in the present invention is to adjust thequantity of light emitted by light-emitting elements in consideration ofthe characteristics of the light-receiving elements. The characteristicsof the light-receiving elements may be registered in the form parametersas a result of factory experiments.

The parameters may be set such that color-to-color variance in thelight-receiving characteristics of an image pickup unit is cancelled.The image pickup apparatus may be provided with a plurality oflight-emitting elements corresponding to respective colors, an imagepickup unit and a light-emitting circuit for causing the light-emittingelements to emit a high-luminance electronic flash light for takingphotographs. The light-emitting circuit may drive the plurality oflight-emitting elements by adjusting the quantity of light to cancelcolor-to-color variance in the light-receiving characteristics of theimage pickup unit.

FIG. 3 shows a construction of a light emission control circuit 500. Theblue light-emitting diode 400 a, the green light-emitting diode 400 band the red light-emitting diode 400 c are connected to switches Swa,Swb and Swc, respectively (hereinafter, these switches may becollectively referred to as switches Sw). The switches Swa, Swb and Swcare connected to transistors Tra, Trb and Trc (hereinafter, thesetransistors may be collectively referred to as transistors Tr). Thetransistors Tr shown are negative-metal oxide semiconductor (N-MOS)transistors. The transistors Tra, Trb and Trc are connected to resistorsRa, Rb and Rc (hereinafter, these resistors may be collectively referredto as resistors R). The transistors Tra, Trb and Trc are connected tooperational amplifiers Aa, Ab and Ac (hereinafter, these operationalamplifiers may be collectively referred to as operational amplifiers A).The non-inverting input of the operational amplifiers Aa, Ab and Ac areconnected to variable constant voltage sources Vca, Vcb and Vcc(hereinafter, these variable constant voltage sources may becollectively referred to as variable constant voltage sources Vc). Theinverting input of the operational amplifiers Aa, Ab and Ac areconnected to the resistors Ra, Rb and Rc, respectively.

A description will be given of a method of controlling flash lightemission, by taking an example of the blue light-emitting diode 400 a.As shown in the figure, one end of the resistor Ra is grounded so thatthe potential thereof is ideally 0. According to the operating principleof the operational amplifier demands, the potential at the other end ofthe resistor Ra is ideally the same as the potential at which thevariable constant voltage source Vca is set. Given that the transistorTra is turned on, a current of a predetermined level is fed to thelight-emitting diode 400 a for emission of light, when the switch Swa isturned on. The green light-emitting diode 400 b and the redlight-emitting diode 400 c are controlled similarly.

A description will be given of various methods, executed in the imagepickup and processing unit 120, of adjusting the quantity of lightemitted by the light-emitting diodes 400 in accordance with thelight-receiving characteristics of the light-receiving elements.

1. A control circuit such as a PWM circuit (not shown) may adjust thequantity of light emitted by the light-emitting diodes 400, bycontrolling the on and off of the switches Sw. Parameters correspondingto the light-receiving characteristics of the light-receiving elementsmay be stored in the memory 214 so that a control circuit (not shown)adjusts the ratio of duration of the on period and off period of theswitches Sw in accordance with the parameters.

2. The current fed to the light-emitting diodes 400 varies in accordancewith the resistance of the resistors R. Accordingly, by adjusting theresistance of the resistors R in accordance with the light-receivingcharacteristics of the light-receiving elements, the quantity of lightemitted by the light-emitting diodes 400 is adjusted.

3. The current fed to the light-emitting diodes 400 varies in accordancewith the voltages set in the variable constant voltage sources Vc.Accordingly, by adjusting the voltage set in the variable constantvoltage sources Vc in accordance with the light-receivingcharacteristics of the light-receiving elements, the quantity of lightemitted by the light-emitting diodes 400 is adjusted.

4. The on and off duration ratio of the transistors Tr may be controlledby controlling the voltages set in the variable constant voltage sourcesVc using a control circuit such as a PWM circuit (not shown). With this,the quantity of light emitted by the light-emitting diodes 400 isadjusted. As mentioned before, the parameters corresponding to thelight-receiving characteristics of the light-receiving elements may bestored in the memory 204 so that the on and off duration ratio of thetransistors Tr is adjusted in accordance with the parameters.

FIG. 4 shows a construction of a light emission control IC 600 in whicha circuit for driving the light-emitting elements is built into onechip. The light emission control IC 600 includes an interface circuit506, an Iref circuit 502 and a Vref circuit 504.

The interface circuit 210 receives a signal from the CPU 212 and outputsa signal to the CPU 212. The Iref circuit 502 and the Vref circuit 504generate a reference current and a reference voltage, respectively. Thevoltage Vref generated by the Vref circuit 504 is input to the erroramplifier 302.

According to the embodiment, the quantity of light emitted by thelight-emitting elements is adjusted in accordance with thelight-receiving characteristics of the light-receiving elements.Particularly, it is to be appreciated that a photographic image true toa subject's original color is taken using an electronic flash as aresult of controlling the quantity of light emitted by thelight-emitting elements corresponding to respective colors, inaccordance with the light-receiving characteristics of thelight-receiving elements corresponding to respective colors.

Described above is an explanation based on the embodiment. Theembodiment of the present invention is only illustrative in nature andit will be obvious to those skilled in the art that various variationsin constituting elements and processes are possible within the scope ofthe present invention.

1. An image pickup apparatus comprising: a plurality of light-emittingelements corresponding to respective colors; an image pickup unit; aboost converter for boosting an input voltage to be supplied to thelight-emitting elements; a memory for storing parameters for drivingsaid plurality of light-emitting elements to emit a high-luminanceelectronic flash light for taking photographs; and a light-emittingcircuit for reading the parameters and driving said plurality oflight-emitting elements in accordance with the parameters, wherein thelight emitting circuit comprises: a plurality of transistors forcontrolling light emission of the plurality of light-emitting elements;a plurality of operational amplifiers having their outputs respectivelyconnected to the gates of the plurality of transistors and having theirinverting inputs respectively connected to the drains of the pluralityof transistors; and a plurality of variable constant voltage sourcesrespectively connected to the on-inverting inputs of the plurality ofoperational amplifiers and controlling the on and off of the pluralityof transistors based on preset voltages determined by the parameters,and wherein the parameters are set so as to correct the light-receivingcharacteristics of said image pickup unit.
 2. The image pickup apparatusaccording to claim 1, wherein the boost converter compares a differencebetween a voltage determined by an output voltage and a referencevoltage with an output value of an oscillator, so as to control theoperation of boosting the input voltage in accordance with a result ofcomparison.
 3. The image pickup apparatus according to claim 1, furthercomprising: an optical sensor unit for sensing ambient light; a processcontrol unit for determining whether emission of a flash light isnecessary by referring to a result of sensing and for driving thelight-emitting circuit.